When a client machine repeatedly sends DHCPDISCOVER messages and receives DHCPOFFER responses but never completes the process with DHCPACK, we're looking at a breakdown in the four-step DHCP handshake (DORA process). This typically indicates either network communication issues or server configuration problems.

The provided dhcpd.conf shows several potential issues that could cause this behavior:

# Key configuration elements to examine:
authoritative;
ddns-update-style none;
deny duplicates;

# Problematic pool declarations:
pool {
  max-lease-time 300;
  range x.x.x.250 x.x.x.254;
  deny known-clients; # This might block valid clients
}

From experience, these are the most likely culprits:

• The deny known-clients in the testing pool might be interfering
• Missing or incorrect subnet declarations
• Firewall blocking UDP ports 67/68
• IP address conflicts in the range

Here's how I would restructure the configuration:

subnet x.x.x.128 netmask 255.255.255.128 {
  option subnet-mask 255.255.255.128;
  option routers x.x.x.129;
  
  # Static hosts pool
  pool {
    range x.x.x.200 x.x.x.220;
    allow known-clients;
    default-lease-time 1800;
  }

  # Dynamic pool
  pool {
    range x.x.x.250 x.x.x.254;
    deny known-clients;
    default-lease-time 300;
  }
}

After making changes:

# Restart DHCP service
sudo systemctl restart isc-dhcp-server

# Check logs for errors
journalctl -u isc-dhcp-server -f

# Verify on client
dhclient -v eth0

If problems persist, use tcpdump to analyze traffic:

sudo tcpdump -i eth0 port 67 or port 68 -vv -n

Look for these sequence patterns: DISCOVER → OFFER → REQUEST → ACK. Missing ACKs will be evident.

When analyzing DHCP traffic, we often expect the standard four-step handshake (DISCOVER-OFFER-REQUEST-ACK). However, in this case, we're seeing a breakdown after DHCPOFFER with no subsequent DHCPACK. This manifests as continuous 30-second retransmission cycles from the client.

The absence of DHCPACK typically indicates one of these scenarios:

• Network connectivity issues preventing packet delivery
• DHCP server configuration problems (especially with pool declarations)
• IP address conflicts with existing leases
• Client-side DHCP service malfunction

The provided dhcpd.conf shows several interesting elements:

# Critical configuration elements
authoritative;
ddns-update-style none;
deny duplicates;

# Fixed host declarations
host host2 { hardware ethernet xx:xx:xx:xx:xx:xx; fixed-address x.x.x.202; }

# Testing pool configuration
pool {
  max-lease-time 300;
  range x.x.x.250 x.x.x.254;
  deny known-clients;
}

1. Verify Server Logs:

tail -f /var/log/syslog | grep dhcpd
# Look for errors like "no free leases" or "declined address"

2. Check Lease Status:

dhcp-lease-list
# Or examine lease file directly
cat /var/lib/dhcp/dhcpd.leases

3. Network Capture:

tcpdump -i eth0 port 67 or port 68 -vv
# Filter for specific client MAC
tcpdump -i eth0 ether host xx:xx:xx:xx:xx:xx and port 67 or 68

The existing configuration can be enhanced with these additions:

# Add to global section
log-facility local7;
one-lease-per-client true;

# Enhanced pool configuration
pool {
  range x.x.x.200 x.x.x.220;
  deny unknown-clients;
  allow known-clients;
  option domain-name "example.com";
  option routers x.x.x.129;
  default-lease-time 1800;
}

On Linux clients, these commands help diagnose DHCP issues:

# Release and renew DHCP lease
sudo dhclient -r eth0
sudo dhclient -v eth0

# Check acquired lease
cat /var/lib/dhcp/dhclient.leases

When dealing with fixed addresses via DHCP:

• Ensure the fixed-address falls within a defined pool range
• Verify there are no overlapping ranges in multiple pool declarations
• Consider using host declarations outside pool blocks for static assignments